The alignof and _Alignof macros return too large values for
the types double and long long in GCC 4.7.0.

Portability problems not fixed by Gnulib:

In ISO C11, the operand of alignof/_Alignof must be a
parenthesized type. Recent versions of GCC support an extension in
which the operand can also be a unary expression, as with
sizeof. The Gnulib substitute does not support this extension.

_Alignas and alignas are not always supported;
on platforms lacking support, the
macro __alignas_is_defined is not defined.
Supported compilers include GCC, IBM C, Sun C 5.11 and later,
and MSVC 7.0 and later.

Some compilers do not support alignment via
alignas/_Alignas of auto variables (i.e.,
variables on the stack). They diagnose and ignore the alignment: Sun
C 5.11.

Some linkers do not support operands of _Alignas/alignas
that are greater than 8: mingw.

Some compilers require the operand of _Alignas/alignas
to be a single integer constant, not an expression: MSVC 7.0 through
at least 10.0.

The Sun C 5.11 compiler sometimes mishandles the alignment of multiple
external variables that are declared close together with
_Alignas/alignas. This compiler bug causes the Gnulib
module stdalign-tests to fail. The Sun Studio Developer Bug
Report Review Team assigned the internal review ID 2125432 (dated
2011-11-01) to this issue.

<stdalign.h> must be #included before _Alignas and
_Alignof can be used.

You cannot assume that _Alignas and _Alignof are reserved words;
they might be macros.